I have a mechanical solution that
might work.
Without your feed in my hands, it's hard to know.
Who knew the darned thing didn't have a removable scalar?
You sure the scalar isn't just stuck in the paint...?
Anyway, it would require fabricating a metal plate about 10-11 inches in diameter, with a big hole in the middle for the feedhorn.
As I was writing it up, I realized a flaw, needing a re-write, and maybe a drawing.
So, I shelved the matter, pending your decision to make-do and fab something, or just get a more friendly feedhorn.
I have their dual-output model, and it can be had probably for less ($75?) than the one you got.